Computer Programming Concepts and Visual Basic David I. Schneider

356 Computer Programming Concepts and Visual Basic
Whenever you double-click on the TextBox icon in the toolbar, a new text box is created.
Although each text box is a separate entity, they all have the same properties and methods.
Each text box is said to be an instance of the class TextBox. In some sense, the TextBox
icon in the toolbox is a template for creating text boxes. (When you look at the properties
window for a text box, the dropdown list box at the top of the window says something like
“Text1 TextBox”. “Text1” is the name of the control object and “TextBox” is the name of its
class.) You can’t set properties or invoke methods of the TextBox icon, only of the specific
text boxes that it creates. The analogy is often made between the TextBox icon and a cookie
cutter. The cookie cutter is used to create cookies that you can eat, but you can’t eat the cookie
cutter.
Object-oriented programs are populated with objects that hold data, have properties,
respond to methods, and raise events. (The generation of events will be discussed in the next
section.) Six examples are as follows:
1. In a professor’s program to assign and display semester grades, a student object
might hold a single student’s name, social security number, midterm grade, and
final exam grade. A SemGrade method might calculate the student’s semester
grade. Events might be raised when improper data is passed to the object.
2. In a payroll program, an employee object might hold an employee’s name,
hourly wage, and hours worked. A CalculatePay method would tell the object to
calculate the wages for the current pay period.
3. In a checking account program, a check register object might record and total
the checks written during a certain month, a deposit slip object might record
and total the deposits made during a certain month, and an account object might
keep a running total of the balance in the account. The account object would
raise an event to alert the bank when the balance gets too low.
4. In a bookstore inventory program, a textbook object might hold the name,
author, quantity in stock, and wholesale price of an individual textbook. A
RetailPrice method might instruct the book object to calculate the selling price
of the textbook. An event could be triggered when the book goes out of stock.
5. In a game program, an airplane object might hold the location of an airplane.
At any time, the program could tell the object to display the airplane at its current
location or to drop a bomb. An event can be triggered each time a bomb
moves so that the program can determine if anything was hit.
6. In a card game, a card object might hold the denomination and suit of a specific
card. An IdentifyCard method might return a string such as “Ace of Spades.”
A deck of cards object might consist of an array of card objects. A ShuffleDeck
method might thoroughly shuffle the deck and a Shuffling event might indicate
the progress of the shuffle.
The most important object-oriented term is class. A class is a template
from which objects are created. The class specifies the properties and methods
that will be common to all objects that are instances of that class. Classes are
formulated in class modules. An object, which is an instance of a class, can be
created in a program with a pair of statements of the form
Private objectName As className ‘In General Declarations section
Set objectName = New className ‘In procedure
In the program, properties of the object is accessed with statements of the form
shown in the following table.